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Quartz crystals in HC37/U packages. - February 2012

Quartz crystals in HC37/U packages.
The HC37/U crystal package is one of the most popular enclosures for high reliability quartz crystals. It is a cold welded enclosure with 5 leads and 4 internal mounts, the 5th lead being a case ground connector. It has an overall diameter at the flange of 13.10mm and comes in 3 heights, 6.65mm, 4.40mm and 3.91mm.
This package is sometimes referred to by the name TO-8.
The â??TOâ? nomenclature comes from a series of package sizes currently used in describing transistors. Among this series are the smaller HC35/U ( TO5 ) and the larger HC40/U ( TO-3 ). There are many other â?? TO â?? sizes but these are the three main ones used in quartz crystal manufacture. The majority of high performance quartz crystals are designed to operate in the HF to VHF bands. ( 3MHz â?? 300 MHz ) TheHC37/U is well positioned as it can operate in several overtone modes covering a range of 3 MHz â?? 150Mz using fundamental through to 5th overtone modes. Higher frequencies can be accommodated using the smaller HC35/U ( TO-5 ) package.
Internally the package can accommodate a quartz blank up to 10.92mm diameter typically mounted on nickel ribbon mounts designed to isolate the quartz element from shock and thermal stress. This diameter is ideal for the popular 10 MHz SC Cut 3rd overtone crystal but surprisingly can also accommodate 3rd overtone SC Cut crystals as low as 5 MHz. This is possible using a polished plano convex blank combined with ribbon mounts that can attach at the very periphery of the blank thus keeping any associated damping to a minimum.
The cold weld technology used to seal it provides the low pressures and low contamination necessary to achieve high Q factors and very low long term stability.
Finally, the generous internal proportions do allow for additional options including internal sensors, directly applied heaters as well as anti-shock mounts.


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